This invention relates to a hydraulic master and slave cylinder assembly and more particularly to such an assembly which is particularly suitable for use in remotely shifting a motor vehicle transmission.
Motor vehicle automatic transmissions are provided with a manual control for selecting the mode of operation of the transmission, for example for shifting from park and/or neutral to reverse or to forward drive, or to prevent automatic shifting from the high gear ratio to a lower gear ratio or to straight through drive, or to overdrive. The manual control or selector takes the form of a steering column mounted pivotal lever, or of a floor mounted lever or quadrant.
The shift control lever or selector conventionally operates, through cables, rods, or links, a mode selection arm pivotally mounted on the transmission casing. The mode selection arm is typically mounted on the end of a control input shaft projecting within the transmission casing and adapted to operate diverse control members within the transmission casing, such as hydraulic valves or solenoids, for functionally establishing an operative mode of the transmission under the control of the shift control or selector lever.
Mechanical control linkages between the shift control or selector lever and the control input arm of the transmission are generally satisfactory but tend to be rather complex in view of the need to tortuously route the various cables, rods and links between the shift control lever and the transmission. In an effort to eliminate the complexity of mechanical shift linkages of this type, it has been proposed to remotely shift the automatic transmission by the use of a hydraulic master and slave cylinder assembly in which the motion of the selector lever is transmitted to the master cylinder and the master cylinder in turn transmits a hydraulic link signal to the slave cylinder which remotely positions the transmission in the desired gear. Master and slave cylinder assemblies of this type are disclosed, for example, in U.S. Pat. Nos. 4,785,615 dated Nov. 22, 1988 and 4,788,821 dated Dec. 6, 1988, both assigned to the assignee of the present application. Whereas hydraulic master and slave cylinder assemblies perform admirably in remotely shifting a motor vehicle automatic transmission, such assemblies are subject to creepage in extreme temperature differential situations and this creepage can actually result in the transmission being inadvertently placed in other than the desired gear or mode. For example, in situations where the master cylinder is located in or adjacent the heated passenger compartment of the motor vehicle, and the slave cylinder is located remotely in the totally unheated area of the motor vehicle, extreme cold weather conditions can produce a situation in which, by virtue of the extreme difference in temperature in which the master and slave cylinders are operating, the output member of the slave cylinder inadvertently creeps relative to the master cylinder to an extent to place the transmission in a mode other than the mode indicated by the selector lever.
Master and slave cylinder assemblies have also been proposed for use with motor vehicle manual transmissions. Such a master and slave assembly suitable for use with a manual transmission is disclosed in U.S. Pat. No. 4,712,640 dated on Dec. 15, 1987 and assigned to the assignee of the present invention. Temperature induced creepage problems can also adversely affect the performance of master and slave cylinder assemblies when used to shift manual transmissions.